LONG-TERM BEACH WIDTH CHANGE OF MONTEREY BAY, MONTEREY BAY NATIONAL MARINE SANCTUARY, CALIFORNIA

The Monterey Bay National Marine Sanctuary is characterized by a variety of shoreline interfaces: rocky headlands, steep cliffs, pocket beaches, and continuous stretches of sandy beaches that define the landward Sanctuary margin. Monterey Bay, situated approximately 100 km south of San Francisco, has 48 km of sandy shoreline that gently curves from Santa Cruz to Monterey. The seasonal oscillation of beach width associated with the transition between narrow winter and broad summer beaches is well constrained. Overprinted on this seasonal signal is a longer, decadal period associated with the El Nino/Southern Oscillation (ENSO) cycle. The effects of ENSO on coastal change are also well documented. However, there are little data quantifying the long-term (century-scale) trends in beach width change. Using historical aerial photography over a 61-year period, and NOS topographic sheets from the 1880s thru the 1970s, a comprehensive spatially continuous, 120-year database of beach width in inner Monterey Bay has begun to shed light on these long-term trends in beach width. Based on this database, beach width of inner Monterey Bay appears to be in steady-state, with a few notable exceptions. In general, the primary mechanism of change in beach width is the positional stability of back beach geomorphic features, which consist of dunes, bedrock cliffs, and poorly consolidated bluffs. We found that the erosional trends in beach width along the inner Monterey Bay are most clearly evident in locations where the back beach feature has been armored to protect existing coastal development. Therefore, by fixing the position of the back beach feature, the (landward) erosional trend of the Bay coastline is interrupted and modified. These perturbations in the apparent steady state configuration of the Bay have had a ripple effect in the system impacting erosion and beach width sustainability in inner Monterey Bay. This database will allow us to quantify the spatial and temporal variability of these anthropogenic perturbations and gage their impact on the long-term sustainability of the beaches in Monterey Bay.